LEDs, as advanced light source technologies in lighting field after incandescent lamps and fluorescent lamps, have been widely applied in various fields such as lighting, display and backlights because of advantages like energy conservation, good durability and pollution-free. LEDs have emerged as a new generation of lighting devices. In existing LEDs, a blue light LED chip or an ultraviolet LED chip together with a light conversion filter for converting light emitted from the LED chip realizes emitting of white light or visible light of other colors.
Indexes for evaluating performance of light emitted from an LED light source include chromaticity, color temperature, light efficiency, and color rendering index, and the like. Among the indexes, chromaticity is a key parameter because chromaticity is a physical color for measuring light itself, especially the light visible to human eyes, and human eyes are most sensitive to color differences, and the second, the brightness of different light (light efficiency), cool and warm of light (color temperature), and authenticity of colors (color rending index). If the chromaticity difference of light is too large, i.e., the dispersion degree of the chromaticity distribution is too large, such difference can be perceived by human eyes. This means that such LED light sources have quality discrepancy, and may have a relatively low yield.
People always expect LED light sources having good chromaticity uniformity and high yield, and they divide LED light sources into different levels according to the chromaticity of the light emitted from the LED sources. When manufacturing LED light sources, one or two levels are defined as qualified levels, and LED light sources within these levels are deemed as qualified products, and a ratio between qualified products and a total number of expected products is defined as the yield. And, the LED light sources outside the qualified levels are deemed as unqualified products.
It has been found from long-term studies that the wavelength of the light emitted from an LED chip significantly influences the chromaticity distribution of an LED light source; one type of phosphor powder can emit light having great differences in light properties under excitation of LED chips emitting light of different light wavelengths, especially the chromaticity difference, thereby resulting in the difference in chromaticity distribution of light emitted from different LED light sources. Because the light emitted from LED chips are usually blue light or ultraviolet light (UV) which has a relatively short wavelength, the light property of such light is hard to detect; if the chromaticity distribution of the light emitted from the manufactured LED chips is directly detected, such detection is not accurate.
At present, LED manufacturers generally improve cutting methods of LED chips in order to reduce wavelength differences of light emitted from manufactured LED chips. In addition, in order to obtain LED light sources having relatively small dispersion degree of chromaticity distribution, a patent application No. CN201110036162.7 discloses a method in which the wavelength of light emitted from an LED chip is evaluated by optical performance of light which is emitted from the LED chip and converted by phosphor powder, and the chromaticity distribution of the light emitted from the LED light source is adjusted by adjusting types of the phosphor powder and contents of the phosphor powder so as to improve the yield.
However, in the above method in which improved cutting scheme is used, the LED chips which are outside the qualified levels are not effectively used. In the above method in which phosphor powder is used as the conversion medium, every LED chip has to be checked and then proper phosphor powder can be prepared, and thus the process for manufacturing LED light sources are complicated and thereby the productivity of LED light sources is restricted.